The DC power supply system normally comprises a battery group, rectifiers and power converters, supplying a high-quality DC (Direct Current) power source to critical loads. In order to promote the quality of DC power supply, the DC power supply system is normally ungrounded, i.e. the positive and negative buses thereof are insulated from the ground. A single ground fault does not arise from the system is forming a short circuit, thereby allowing it to still power to the loads. Importantly, power supply systems stop working when a ground insulation fault occurs in two or more points.
The balanced bridge detection method is a conventional means of detecting the insulation fault of ungrounded DC power supply systems, in which the ratio of the insulation resistance to ground of the positive bus to that of the negative bus is generally set as 2:1˜10:1. A ground insulation fault refers to a situation in which ratio of the voltage to ground of the positive bus and that of the negative bus is higher than the above setting value. Although the positive bus and the negative bus significantly differ in the insulation value, the insulation resistance values of the positive and negative buses are not lower than that of the permitted value; the balanced bridge detection method may still create false alarms.
The unbalanced bridge detection method is a modification of balanced bridge method. This method is applicable for the positive and negative bus when their insulation resistance falls are equal, and can also detect the insulation resistance value of the positive and negative bus to ground. However, this method cannot identify which branch circuit has a ground fault, and so each branch circuit requires a branch leakage current detector. The positive and negative bus must be fitted with a resistor when using the unbalanced bridge method to detect the fault, reducing the insulation of the DC power system.
AC signal source injection method: The basic principle of this method is to inject a low frequency AC signal source in the positive and negative bus, and then use an AC current sensor to detect the low frequency AC current signal of the branch circuit. The ground resistance can then be calculated based on the magnitude and phase angle of this current. When the measured resistance value is below the set value, this branch circuit has a ground fault. Due to micro electro mechanical devices recently using numerous anti-interference capacitors as a transient voltage compensator, this increases the capacitance in the DC power supply system and results in larger capacitance current. AC signal source injection to detect ground fault point thus is ineffective for ground fault detection of a branch circuit. That is, when the ground capacitance current exceeds the leakage current value of the standard of insulation resistance, it generates a false alarm that affects the correct determination of the insulation false detection device.
Theoretically, a single point ground fault does not produce a leakage current in ungrounded DC power systems. However, DC power systems may have some grounded stray capacitors and external capacitors, thereby providing some paths for ground fault leakage current. Notably, the paths for leakage current are closed when the charges of the capacitors reach steady state. Correspondingly, deterioration of the insulation resistors can be detected via the transitory leakage current paths provided by the grounded capacitors and detection of the difference of the currents of the positive and negative buses.
With a deteriorating insulation resistance of the positive and negative buses, their voltages to ground change as well. The bus with lower ground resistance has a lower voltage to ground; the bus with a higher ground resistance has a higher voltage to ground. When the positive bus insulation resistance deteriorates, the initial value of leakage current IGN+ (t0) is equal to the voltage to ground of the positive bus Vc (t0) divided by the insulation resistance to ground of the positive bus RN+, IGN+(t0)=Vc(t0)/RN+. The leakage current decreases to zero after the charges of grounded capacitors achieve the steady state. Thus, if grounded capacitors are used to detect a leakage current, the leakage current must be measured immediately when the insulation resistance changes. The leakage current decreases exponentially and, then, approaches zero after a period of time.
Insulation resistance of the power system does not rapidly deteriorate. Generally, these insulation resistors deteriorate gradually, and the aging speed is extremely slow. When the voltage of the fault bus to ground is inadequate, the leakage current is too low to drive the leakage current detection device. Thus, the ground insulation fault is undetected. In this case, the grounded capacitors method can not detect the ground insulation fault successfully.
For example, consider a 100 V ungrounded DC power supply system, in which the current detection sensitivity is 1 mA; a ground fault alarm signal appears after the insulation resistance decreases below 50 kΩ. Initial values of the insulation resistance of positive and negative buses to ground are 500 kΩ and, then, the voltages of positive and negative buses to ground are 50 V. In this case, if the insulation resistance of positive or negative buses breaks down suddenly, the insulation resistance decreases rapidly from 500 kΩ to 49 kΩ; the initial leakage current is IGN+(0)=50 V/49 kΩ=1.02 mA; and the system creates a fault alarm signal.
The above example reveals that, if the insulation resistance of the positive bus to ground gradually decreases, its voltage to ground also decreases slowly. When the insulation resistance of the bus to ground gradually decreases to 51 kΩ, its voltage to ground also decreases slowly to 9.26 V (100/(500+51)×51=9.26 V). After the insulation resistance of the bus decreases further to 49 kΩ, the system should have created an alarm signal because the insulation resistance is already lower than the fault threshold value of 50 kΩ. However, as the insulation resistance is reduced from 51 kΩ to 49 kΩ, the initial leakage current IGN+(0) is only 0.19 mA (IGN+(0)=9.26 V/49 kΩ). Such a leakage current is too low to drive the current sensor with 1 mA sensitivity. This implies that the grounded capacitors method cannot accurately detect the ground insulation fault when the insulation resistance slowly decreases. To compensate for the inability of the current sensor to accurately detect the leakage current when the insulation resistance slowly decreases, this invention proposes a novel DC current injection method, capable of significantly improving the ground insulation fault detection ability of ungrounded DC power supply systems.